The role of cell mediated immunity is believed to be an important component of the specific immunologic response particularly in viral infections. It has been shown that intracellular replication and cell to cell transfer of virus can occur in spite of high titers of circulating antibody. The precise mechanism by which cell mediated events are involved normally in viral immunity is still not clear. It would appear, however, that some cell mediated events are particularly important for infections which lead to a chronic viral disease. This is especially true with congenital disease. It may be that diseases become chronic due to a deficiency in the cell mediated response. The development of in vitro assays for testing delayed hypersensitivity in chronic viral diseases will be attempted. There appears to be a "blocking factor" present in some SSPE spinal fluids. Attempts will be made to characterize the factors and follow effects of treatment on this factor. Cellular immune studies are being extended with MS patients.